1. Field of the Invention
The present invention relates to an electro-optical modulator. More particularly, the present invention relates to an electro-optical modulator with a curving metal-oxide semiconductor (MOS) device.
2. Description of Related Art
Signal transmission is expected to be changed from previous long-distance transmission, middle-distance transmission, and current module-to-module transmission to chip-to-chip transmission and intra-chip transmission in the future. Conventional cable transmission will be confronted with bottleneck along with the increases in both transmission speed and bandwidth. To meet such requirement, the characteristic of high transmission speed of optical signal has to be integrated with module-to-module, chip-to-chip, or even intra-chip transmission through optical waveguide.
In the future, if the processing speed of an intra-chip communication product in an integrated circuit (IC) is increased up to certain degree, for example, the operation speed of a central processing unit (CPU) reaches 5 GHz, then the internal transmission interface in the IC has to reach certain speed correspondingly. The previous low-cost copper cable design is confronted with bottleneck in the design of high frequency signal transmission. The design and fabrication of such high frequency circuit is very complex and has high cost, thus, copper cable loses its advantage in such circuit design.
Accordingly, electro-optical system chip is one of the major techniques to be developed, wherein an electro-optical modulator of high speed, small volume, and CMOS process compatibility is further required to meet the requirement of intra-chip communication. Usually a semiconductor electro-optical modulator changes the resonant property of a resonator corresponding to an operating wavelength by controlling free carriers of a semiconductor, so as to serve as an optical switch and accordingly to transmit digital signals quickly.
FIG. 1 is a cross-sectional view illustrating the semiconductor structure of a conventional electro-optical modulator. Referring to FIG. 1, a conventional electro-optical modulator is composed of a P-I-N diode. A silicon oxide insulating layer 102 is formed on a silicon substrate 100. A silicon layer 104 having a rib thereon is formed on the insulating layer 102 to serve as a resonator. An N-doped region 106 and a P-doped region 108 are respectively disposed at two sides of the rib, and an operation voltage is supplied to the two by a cathode 112 and an anode 114. Besides, a silicon oxide layer 110 covers the exposed silicon layer 104 between the cathode 112 and the anode 114. The structure illustrated in FIG. 1 adopts a diode as the base thereof, and dopants corresponding to the conductive types of the N-doped region 106 and the P-doped region 108 are respectively doped into the silicon layer 104 to form the diode. The optical resonator is provided by the silicon layer 104 having a rib. The mechanism of the electro-optical modulator described above is achieved by controlling free carriers of the semiconductor. However, the running speed of free carriers is very slow, so that the speed of the electro-optical modulator is lower than 1 GHz. An improved design has been provided in U.S. Pat. No. 6,298,177 to increase operation speed.
FIG. 2 is a cross-sectional view illustrating the semiconductor structure of a conventional annular electro-optical modulator. Referring to FIG. 2, another design of electro-optical modulator is described in pages 325-327 of NATURE Vol. 435 issued on 19, May 2005, wherein a micro-ring resonator is used as the base of the electro-optical modulator. A linear waveguide 120 has an input terminal and an output terminal. An optical signal is input from the input terminal. An annular resonator and the linear waveguide 120 have an optical coupling region which can lead the input optical signal having operating wavelength into the annular resonator. The transverse structure of the annular resonator is a resonant silicon layer having three regions. The resonant silicon layer has an annular rib 122 in the middle to achieve resonant effect along with the resonant mode. An N-doped annular region 126 is disposed at the external rim of the rib 122 to serve as a cathode. A P-doped region 124 is disposed at the internal rim of the rib 122 to serve as an anode.
The operation mechanism illustrated in FIG. 2 is to modulate an optical signal through the resonant effect to a particular wavelength. When a suitable voltage is supplied, the resonant wavelength function changes along with the voltage level, wherein the characteristic resonant wavelength also shifts. Thus, the transmittance of an input light having fixed wavelength in the linear waveguide 120 changes along with the resonant state so that the effect of an optical switch is achieved.
However, the electro-optical modulators described above are both based on the design of Mach-Zehnder interference, thus, the lengths thereof, which are usually in mm level, are very long. Accordingly, the device has very large volume and is very difficult to be applied to intra-chip communication. An electro-optical modulator has to be high-speed, small-volume, and CMOS process compatible to meet the requirements of intra-chip communication. Moreover, such performances as high speed, small volume, and CMOS process compatibility of an electro-optical modulator are the concerning of future development.